In a wireless communication system, channel state information (CSI) refers to known channel properties of a communication link between a transmitter and a receiver. The CSI describes how a wireless signal propagates from the transmitter to the receiver and represents the combined effect of multipath scattering, fading, and power decay with distance. The CSI may be determined through a method of channel estimation. The CSI assists in adapting communication to current channel conditions, which is important for achieving reliable communication with high data rates, particularly in multi-antenna and OFDM systems. The method of determining CSI is important for proper symbol detection in a receiver within a modem chipset. Since the channel is time varying, the CSI needs to be provided for each symbol in a transmitted frame. The accuracy of the channel estimation significantly influences the performance of the receiver.
In a wireless communication system such as wireless fidelity (WiFi), the channel may not vary over time or vary slowly due to the low mobility of the devices connected to the system. In the IEEE 802.11 specifications (such as IEEE 802.11n and IEEE 802.11ac), the initial channel estimation which generates CSI is obtained using long-training field (LTF) symbols. The initial channel estimation may be used to demodulate the data symbols transmitted after the LTF symbols. For each of the data symbols transmitted in the frame after the LTF, symbol detection is performed using the CSI from the previously detected data symbol, and the previously detected data symbols are re-used to estimate the channel which refines the channel estimation obtained initially using the LTF. The noise is averaged, and the channel is tracked due to time varying channel conditions. This process used to re-estimate the channel and to update the CSI accordingly is referred to as channel tracking. The channel tracking may be performed across the entire frame for each symbol in an iterative fashion.